The present invention relates to an air intake system for internal combustion engines, in which the intake air amount can be increased over a wider operating range by making use of the dynamic effect of an intake air column, and in which a successful combustion state can be obtained by generating longitudinal vortices (tumbles) that flow axially of the cylinder even in low-intake operating regions where the amount of intake air is small.
Conventionally, the so-called inertia supercharging method in which the intake air amount is increased by making use of the dynamic effect of an intake air column has been adopted in the reciprocating internal-combustion engine from the viewpoint of output enhancement.
In this inertia supercharging, the relationship among pipe length l, pipe cross-sectional area A and an engine speed N at which inertia supercharging can be obtained can approximately be expressed by the following Helmholtz resonator vibration equation: EQU N.sub.rpm =K.multidot.aA.multidot./l.multidot.V.sub.m
where V.sub.m is the cylinder internal effective volume, a is the acoustic velocity and K is the proportional constant. From this equation, it can be easily understood that the inertia supercharging can be achieved by setting the pipe cross-sectional area A or the pipe length k to a value corresponding to the engine speed.
On the other hand, the variable-speed engine, particularly automobile engine, varies in working engine speed over a wide range from idle state to maximum engine speed. Therefore, in automobile engines employing the conventional inertia supercharging, the so-called tuning point at which the intake air amount is increased by inertia supercharging is set to around the engine speed with which the maximum torque or maximum output is generated. As a result, this type of conventional engine has been such that increase in torque or output could not be expected in regions other than the tuning point as a matter of course.
This being the case, as an air intake system capable of obtaining the inertia supercharging effect in a plurality of engine speed regions, there has conventionally been developed a system in which, for example, two air inlet pipes are arrayed in parallel, one inlet pipe being equipped with an opening/closing valve which is opened and closed responsive to the engine speed.
In this air intake system, because the inlet pipe cross-sectional area can be switched between two levels responsive to the engine speed, higher torque can be attained in both high- and low-speed regions.
Also for improvement in the combustion state of engines, it is known that generating longitudinal vortices, i.e. so-called tumbles that the intake air flows along the cylinder axis within the cylinder is effective. A conventionally known structure for generating these tumbles is, for example, one in which the cross section of the air intake path is shaped into a generally inverted triangle.
However, in the conventional system in which a plurality of systems of air intake paths are arranged in parallel, there is a problem that a large space is required in proportion to the parallel arrangement of a plurality of air inlet pipes. In particular, for a multiple cylinder engine, which is limited in intervals between the cylinders, it is difficult to actualize the arrangement of a plurality of systems as described above, while its structure tends to become complex.
Also, for the conventional system in which the cross section of the air intake path is shaped into a generally inverted triangle, it is difficult, as it stands, to reliably generate the tumbles because of low intake air flow rate in low-intake operating regions of small intake air amount, such as low-engine-speed, low-load operating regions. With an arrangement that the vertex of the inverted triangle is made smaller for reliable generation of tumbles even in the low-intake operating regions, conversely, there would arise a problem that the amount of air intake at high engine speed becomes insufficient to obtain high output.
An object of the present invention is to provide an air intake system for internal combustion engines, which solves the foregoing problems, and which is capable of actualizing the switching of the tuning point between two levels almost without causing any increase in the installation space, capable of increasing the intake air amount over a wide range of engine speed regions, and capable of improving the combustion state by positively generating the tumbles even in the low-intake operating regions.
In order to achieve the above object, the present invention provides an air intake system for internal combustion engines in which combustion air is led into a cylinder via an air inlet path, characterized in that: the air inlet path comprises a double structure of a main inlet path and a sub inlet path that the sub inlet path is placed in close contact with a bottom wall of the main inlet path, a downstream end of the sub inlet path being positioned near an inlet valve opening on its one side other than an exhaust valve opening side; and that an opening/closing valve is disposed within the sub inlet path, wherein the opening/closing valve is closed in a low intake-air amount operating region of the engine so as to allow intake air to be biased from the main inlet path toward a center of the cylinder and to flow into the cylinder along a cylinder axis.